This invention relates to the field of signal isolators, and more particularly, to signal isolators that use magnetic fields to provide electrical isolation between first and second electrical signals.
Many electronic applications require some form of signal isolation. Some of these applications include, for example, industrial control applications, down-hole petroleum and geothermal applications, certain medical applications, and numerous other applications. Signal isolators can be used, for example, to couple signals that have relatively large offsets in DC voltage levels, to prevent ground loops for reducing system noise, etc.
One approach for providing signal isolation is through the use of one or more opto-couplers. Opto-couplers use light to couple two electrically isolated circuits. One opto-coupler approach is shown in U.S. Pat. No. 5,946,394 to Gambuzza. A limitation of using opto-couplers is that special hybrid fabrication techniques are often required, which can increase the cost of such devices.
Another approach for providing signal isolation is through the use of one or more Carrier-Domain-Magnometers (CDMs). In this approach, a magnetic field is used to couple two electrically isolated circuits. U.S. Pat. No. 4,849,695 to Muller et al. and U.S. Pat. No. 4,801,883 to Muller et al. each show a signal isolator that uses one or more Carrier-Domain-Magnometers (CDMs). CDMs are typically npnp or pnpn devices that are manufactured directly into the substrate of an integrated circuit. A limitation of many CDMs is that they often consume significant real estate, draw significant power, have limited sensitivity, and may be relatively slow.
The present invention overcomes many of the disadvantages of the prior art by providing a signal isolator that uses one or more magneto-resistive magnetic field sensors. Magneto-resistive magnetic field sensors may be less expensive to manufacture than opto-couplers, and may consume less real estate (particularly since they can often be stacked above electronic circuitry), draw less power, have higher sensitivity and may be faster than CDM based sensors.
In a first illustrative embodiment of the present invention, a signal isolator is provided that has an input coil, a magneto-resistive magnetic field sensor, an output or feedback coil, and a feedback circuit. The input coil receives an input signal and generates a corresponding input magnetic field at the magneto-resistive magnetic field sensor. The magneto-resistive magnetic field sensor senses the input magnetic field and provides a corresponding output signal. The feedback circuit receives the output signal from the magneto-resistive magnetic field sensor, and provides a feedback signal to the output coil such that the output coil generates an output magnetic field that at least substantially nulls out the input magnetic field. An isolated output signal that is related to the input signal can then be derived from the feedback signal.
An advantage of this configuration is that the transfer characteristics of the signal isolator are relatively independent of the sensitivity of the magneto-resistive magnetic field sensor. It is known that the sensitivity of magneto-resistive magnetic field sensors often depends on a number of factors including, for example, temperature, voltage, or just the mere passage of time (drift). However, because the changes in sensitivity of the magneto-resistive magnetic field sensor are experienced equally by the transduced input and output (feedback) magnetic fields (i.e., common mode), the signal isolator may be relatively insensitive to such sensitivity changes. Accordingly, it is contemplated that the illustrative signal isolator may be used in relatively harsh environments such as high temperature environments where sensitivity of the basic sensor changes significantly, high radiation environments where devices using semiconductor junctions are damaged by high energy particles, etc.
In another illustrative signal isolator of the present invention, two magneto-resistive bridge sensors are used, each operating at a different characteristic point along the applied field versus output voltage curve (e.g., two different input magnetic fields produce two different output signals). By using such a configuration, changes in the sensitivity of the bridge sensors can be detected. Once detected, the excitation voltage that is applied to the bridge sensors can be adjusted to compensate for any changes in sensitivity by maintaining a predetermined offset between the two signals, such as a constant value.
In another illustrative embodiment, only a single magneto-resistive bridge sensor is used to sequentially perform both measurements of the first and second magneto-resistive bridge sensors of the previous illustrative embodiment. In this embodiment, the input signal is provided to an input coil, and the resulting magnetic field is sensed by the magneto-resistive bridge sensor to provide a first output signal, which is subsequently stored. Thereafter, a bias signal (e.g., bias current) is added to the input signal to provide an offset input signal. The offset input signal is then provided to the input coil. The magneto-resistive bridge sensor then senses the resulting input magnetic field and provides a second output signal. The second output signal can be stored, or alternatively, directly compared to the first output signal. A compensation circuit then updates the excitation voltage that is provided to the magneto-resistive bridge sensor so that the difference between the first output signal and the second output signal remains at a predetermined value, such as a constant value.
In another illustrative embodiment, an AC source is provided for modulating either the input signal itself or the input magnetic field. The magneto-resistive bridge sensor senses the modulated magnetic field, and provides a modulated output signal. The modulated output signal is provided to a filter, which filters out the higher frequency modulation component provided by the AC source and leaves the lower frequency component provided by the input signal.
To provide compensation, a peak-to-peak detector 524 detects the peak-to-peak voltage of the modulated output signal of the magneto-resistive bridge sensor. The peak-to-peak voltage is compared to a predetermined reference, and the bridge excitation voltage is adjusted until the peak-to-peak voltage equals the predetermined reference voltage 526.